The present invention relates to a fire alarm system and, more particularly, to a fire alarm polling system.
A conventional smoke sensor is operated when an analog sensor output corresponding to a smoke concentration exceeds a predetermined value. In a conventional fire alarm system, a plurality of smoke sensors of the type described above are separately connected to a transmission line. When any one of the smoke sensors detects a fire, this smoke sensor is operated to short circuit the transmission line, thereby signalling the existence of a fire to a main unit (central receiver) connected to the transmission line. However, the sensitivity of smoke sensors often varies, so that an erroneous alarm is generated or no alarm is generated. For example, even if an erroneous alarm or the like is prevented by logic ANDing or ORing the outputs from two smoke sensors, sufficient reliability cannot be obtained. Furthermore, in another conventional system, a sensor terminal does not determine whether or not a fire has broken out, but transmits to a main unit a digital signal obtained by converting an analog sensor output. The main unit compares the digital signal with a predetermined value so as to detect whether or not a fire has broken out. In this case, it is difficult for the main unit to perform detection in consideration of variations in the sensitivity of the smoke sensors.